J. Dahl et al., EVIDENCE OF A ROLE FOR PHOSPHATIDYLINOSITOL 3-KINASE ACTIVATION IN THE BLOCKING OF APOPTOSIS BY POLYOMAVIRUS MIDDLE T-ANTIGEN, Journal of virology, 72(4), 1998, pp. 3221-3226
A polyomavirus mutant (315YF) blocked in binding phosphatidylinositol
3-kinase (PI3-kinase) has previously been shown to be partially defici
ent in transformation and to induce fewer tumors and with a significan
t delay compared to wild-type virus. The role of polyomavirus middle T
antigen-activated PI 3-kinase in apoptosis was investigated as a poss
ible cause of this behavior. When grown in medium containing 1D-3-deox
y-3-fluoro-myo-inositol to block formation of 3'-phosphorylated phosph
atidylinositols, F111 rat fibroblasts transformed by wild-type polyoma
virus (PyF), but not normal F111 cells, showed a marked loss of viabil
ity with evidence of apoptosis. Similarly, treatment with wortmannin,
an inhibitor of PI 3-kinase, stimulated apoptosis in PyF cells but not
in normal cells. Activation of Akt, a serine/threonine kinase whose a
ctivity has been correlated with regulation of apoptosis, was roughly
twofold higher in F111 cells transformed by either wild-type virus or
mutant 250YS blocked in binding Shc compared to cells transformed by m
utant 315YF. In the same cells, levels of apoptosis were inversely cor
related with Akt activity. Apoptosis induced by serum withdrawal in Ra
t-1 cells expressing a temperature-sensitive p53 was shown to be at le
ast partially p53 independent. Expression of either wild-type or 250YS
middle T antigen inhibited apoptosis in serum-starved Rat-1 cells at
both permissive and restrictive temperatures for p53. Mutant 315YF mid
dle T antigen was partially defective for inhibition of apoptosis in t
hese cells. The results indicate that unlike other DNA tumor viruses w
hich block apoptosis by inactivation of p53, polyomavirus achieves pro
tection from apoptotic death through a middle T antigen-PI 3-kinase-Ak
t pathway that is at least partially p53 independent.